1. Field of the Invention
The present invention relates to a cutting tool coated with a hard coating exhibiting excellent adhesion resistance and excellent wear resistance and to a tool with a holder incorporating the same.
2. Description of the Related Art
Conventionally, an arc-ion-type PVD process has been frequently employed for PVD coating with a TiAlN material (coating through physical vapor deposition), since high energy is required for ionization of a TiAl metal and since the process has the advantage, among others, that a coating layer of excellent adhesion is readily obtained.
However, in some cases, this process permits the generation of coarse TiAl grains which is caused by a local concentration of arc derived from the non-uniform surface condition of a TiAl target or a non-uniform generation of arc. As a result, the deposition of coarse TiAlN grains occurs.
Some coarse grains have a size of several micrometers to several tens of micrometers. When such coarse grains are present on the surface of a coating that is provided on a cutting tool, the following problem is encountered: contact of the surface with chips from a work material, mechanical vibration, or the like may cause the coarse grains to drop off of the tool. Exposure of a substrate that results from the coarse grains dropping off from the tool has sometimes caused unstable cutting performance (e.g., adhesion resistance and wear resistance), and in the worst case impairs the cutting performance.
To cope with the above problem, PVD apparatus manufacturers and cutting tool manufacturers have investigated coating conditions and coating processes that can eliminate coarse grains. At present, to reduce the generation of coarse grains, a load current to a target is decreased, a trap for eliminating the coarse grains is provided between a target and a substrate to be coated, or a similar method is employed.
However, the above-mentioned methods impair productivity and increase costs since the coating deposition rate drops.
Some coatings provide improved surfaces in which the generation of coarse grains is prevented through employment of a multilayer structure consisting of TiN, AlN, etc. However, these multilayer structures fail to sufficiently exhibit inherent properties of the TiAlN film in relation to heat resistance, wear resistance, etc., and thus the performance thereof is not satisfactory.
As described in Japanese Patent No. 2901043, an improvement in performance is attempted by eliminating coarse grains after coating to thereby establish a surface having craters formed thereon. However, this technique involves the following practical problem. In some cases, the substrate may be exposed at a crater portion of the coating. When a work material adheres to the crater portion through fusion, and the crater portion is exfoliated, a surrounding portion of the hard coating or the substrate is chipped off together with the adhered work material. As a result, unusual wear is initiated from the chipped portion, and thus satisfactory performance is not necessarily exhibited. Further, since a process for eliminating coarse grains must be carried out after coating, productivity is impaired and an increase in cost results.